1. Field of the Invention
The present invention relates to an apparatus for drying a moist particulate material having a non-uniform particle size with superheated steam, which apparatus comprises a cylindrical vessel comprising a number of parallel, substantially vertical elongated chambers located in ring form, one or more of the chambers having a closed bottom and the remaining chambers having a steam-permeable bottom, the adjacent chambers being interconnected through openings in the chamber walls at the lower ends of the chambers and the upper ends of the chambers being connected with a transfer zone, means for supplying moist particulate material to a chamber having a steam-permeable bottom, means for discharging dried material from a chamber having a closed bottom, means for supplying superheated steam to the area below the steam-permeable chamber bottoms, means for discharging steam from the transfer zone, and means for reheating the discharged steam and recirculating it to the area below the steam-permeable chamber bottoms.
2. The Prior Art
An apparatus of the type mentioned above is disclosed in EP patent specification No. 0153704. This prior art apparatus is particularly suitable for drying beet pulp formed by extracting sugar from sugar beet slices with water, but the apparatus is also suitable for removing liquid, including other liquids than water, from a number of sensitive organic materials.
The prior art apparatus presents the advantage that the particulate material is dried without the access of air, thereby making it possible to avoid oxidation of the material during drying. Another important advantage of the apparatus is that it is environmentally highly acceptable as the drying takes place in a substantially closed system. Furthermore, the excess amount of steam, which, e.g., is generated when drying beet pulp, is very pure and consequently it can be used for the concentration of sugar juice, and the condensate thus formed does not cause odor nuisances as compared to the emission products formed by, e.g., drum-drying beet pulp.
When drying particulate, water-containing organic materials it is important to obtain a high dry matter content, e.g. above 90%, in all parts of the material to ensure the storability of the dried material.
In other words it is not sufficient to obtain a material which on the average has a high dry matter content.
When drying particles having different particle sizes in an apparatus of the type mentioned above, the lightest particles are preferably dried in the upper portion of the drying chambers, whereas the coarse particles primarily are dried at the bottom of these chambers while moving from chamber to chamber via the holes in the lowermost portion of the chamber walls.
In practice it has been found that the coarse part of the material sometimes passes so quickly through the prior art apparatus that part of the large particles is insufficiently dried before reaching the discharge chamber. It has been attempted to solve this problem by increasing the ratio of steam to moist material but this has resulted in an undesired increase in energy consumption.
It has also been attempted to increase the retention time of the material in the drying chambers by reducing the holes in the chamber walls in the flow direction of the material, but this has increased the risk that the material forms coatings on the walls of the drying chambers and that the flow of material is gradually blocked.
The object of the present invention is to obtain a dried particulate material wherein substantially all particles have obtained a desired high dry matter content at a moderate energy consumption.